The phenomenon of superconductivity which many metals exhibit at low temperatures is of great scientific and commercial value since it permits various high powered devices to operate with minimal losses of electrical power. Superconducting devices can be used in such commercial applications as motors, generators, transformers, power lines, medical imaging systems, and large scale supermagnets. Several metal alloys have been discovered which exhibit superconducting properties and have been used to produce multifilament superconducting composites. These superconducting materials include Nb.sub.3 Sn, V.sub.3 Ga, Nb-Ti, and Nb.sub.3 Al. These superconducting materials are typically combined with a normal conductive metal and then formed into long, thin filaments to produce a multifilament superconducting/normal metal composite.
One method for preparing a multifilament superconducting wire, the so-called "bronze process") having a superconducting material such as Nb.sub.3 Sn, consists of drilling a plurality of evenly spaced holes in a copper or bronze billet, inserting niobium rods in each hole, and extruding and drawing the billet in several steps until the niobium rods are reduced to the desired filament size. The wire is then coated with tin and heated to react the tin and niobium in order to form the Nb.sub.3 Sn superconducting material. However, this process is expensive and exacting and generally limits the filament size to filaments larger than 2 .mu.m in diameter.
Another method for preparing a multifilamentary superconducting wire, the so-called in-situ process, involves casting large billets of Cu and Nb together in a consumable arc casting process. This produces a casting in which a dense array of Nb dendrites about 6 .mu.m in diameter are dispersed in a Cu matrix. This billet is drawn to wire, coated with Sn, and heat treated to transform the Nb to Nb.sub.3 Sn. These types of composite superconducting wires are known as in-situ composites because the superconducting filaments are formed in place during the process of preparing the multifilament superconducting composite.
Thus, neither of the prior art methods of forming superconducting wires described above separately prepare the filaments of the superconducting material prior to combining them with a normal metal material. Rather, as described above, the filaments are prepared during the casting process.
As stated above, multifilamentary superconducting composites have played a central role in the development of conductors for commercial applications such as large scale magnets because they are more stable magnetically than monolithic tapes and they are far easier to handle in the process of winding the magnet. The fabrication of these composites for high critical temperature materials such as those set forth above, however, has been problematic because the materials are so brittle and the chemistry so complicated.
Advances recently have been made in the development of high temperature superconducting materials based on copper-bearing oxides such as Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 and various compositions of Bi-Sr-Ca-Cu-O. These materials have been processed using a wide variety of techniques in an attempt to produce useful engineering devices. Some of the processing techniques used include plasma spraying, sputtering, and laser-heated pedestal growth. However, because of the complexity of these processing techniques, none have been found feasible for use on a mass production basis.
Other methods for processing these materials in filament form have been developed which are more feasible for use on a mass production basis. These methods include a pendant drop melt extraction process and a gas jet fiberization process. The gas jet fiberization process for making amorphous Bi-Sr-Ca-Cu-O fibers has thus far proved to be the most successful. With the gas jet fiberization process, liquid drops of Bi-Sr-Ca-Cu-O are directed through a supersonic nozzle where a gas stream shapes and freezes the liquid into filaments approximately 1 cm long having diameters that range from 0.5 to 3 .mu.m. The resulting filaments are in an amorphous state, and must be converted to a superconducting state in a subsequent process in order to produce multifilament superconducting wires. As can be seen, the gas jet fiberization process is an ex-situ process, since the filaments are formed separately and must separately be combined with a normal metal material subsequent to their formation.
Although superconducting filaments formed from Bi-Sr-Ca-Cu-O show promise for use in multifilament superconducting wires, several problems exist which must be overcome. For example, superconducting filaments composed of Bi-Sr-Ca-Cu-O cannot feasibly be produced with the types of in-situ production methods discussed above. A significant reason for this is the complex chemical composition of these filaments. Therefore, new methods must be developed for using these separately formed filaments to produce superconducting wires. Further, with the gas jet fiberization and pendant drop melt extraction methods, the resulting filaments are in an amorphous state and must be converted to the superconducting state by heat treatment. Control of the conversion of these amorphous filaments to the superconducting state is important, because the filaments can coarsen during heat treatment which destroys the long slender aspect ratio of the filaments. Finally, because of the geometry of the filaments, that is, their comparatively short length, on the order of 1 cm, and their resulting discontinuous nature, it is difficult to realize large supercurrents in a superconducting wire having only a relatively small number of these filaments. Thus, a method of using a large number of these discontinuous filaments must be developed which provides substantial filament-to-filament transfer of supercurrents among the discontinuous filaments.
In spite of these difficulties, the use of Bi-Sr-Ca-Cu-O filaments has many potential advantages. For example, with the gas jet fiberization process, the resulting ex-situ filaments are amorphous, electrically insulating and relatively strong. The flexibility of these filaments, because they are in an amorphous state, allows them to better withstand mechanical processing to form microfilamentary superconducting wires. Further, the use of Bi-Sr-Ca-Cu-O filaments has great advantages over the use of filaments formed from other superconducting materials such as those set forth above. For example, the superconducting materials discussed above exhibit poor mechanical properties, namely brittleness, and are not as reliable for use in commercial applications. As a result, these other superconducting materials are not as suitable for production on an industrial scale. Therefore, if the problems discussed above can be overcome, it may be possible to use the flexible Bi-Sr-Ca-Cu-O filaments for producing superconducting wires on a mass production basis.